Measurement of prostate specific antigen as screening test for prostate cancerBMJ 1996; 312 doi: https://doi.org/10.1136/bmj.312.7032.708a (Published 16 March 1996) Cite this as: BMJ 1996;312:708
False positive results cast doubt on conclusion
EDITOR,—Carol Parkes and colleagues' main conclusion that 60-74 year old men with a prostate specific antigen concentration >/=12 times the normal median value have about a 50% chance of developing clinical prostate cancer within three years seems speculative and unsupported by the data presented.1 We are told that over the whole age range of the 49000 men studied 16 developed prostate cancer within three years of a blood sample being obtained, of whom 13 would have been detected if >/=12 times the normal median concentration of prostate specific antigen was used as the cut off value. With this cut off 0.5% of the men—that is, about 245 (on the assumption that serum samples were available for most subjects)—would have yielded false positive results, so someone with a positive result on screening would have a 1 in 19 chance of developing clinical prostate cancer within three years. The authors do not mention this but instead invoke calculations based on population based data (not from this study) to argue that looking at men aged 60-74 would give the much more impressive specificity quoted in their conclusion.
We must speculate why we are not told how many of the men aged 60-74 who were studied had >/=12 times the normal median concentration of prostate specific antigen but did not get prostate cancer. The investigators must know these data, and omitting them severely damages the credibility of the conclusion.